Router Template Guide Bushings - Why So Many Sizes?

This is going to be a real rookie question, so please be gentle.
I've never used router template guide bushings before, but I am about to.
To install this bed rail hardware, I need to make a 5/8" mortise for the slotted portion and then two 1/4" mortises right in the center of that mortise to accept the hooks. My testing shows that a single pass with a 5/8" bit makes a nice snug mortise for the hardware.
http://www.cshardware.com/15-74-5.html
It seems to me that if I make a jig to accept the bushing for the 5/8" bit, then all I need to do is swap out the bit and the same jig can be used for both mortises. The 1/4" bit will be centered in the 5/8" mortise "by default". That seems pretty straight forward.
So assuming that I am OK with using that technique, here's my question:
Why are there so many different sized bushings? Why not have just 1 bushing to accept the largest bit and then make all jigs to work with that one bushing? All bits, regardless of their size, will be centered in the bushing, so why are there multiple bushings that match the bits when its the *jig* that determines where the bit will contact the wood?
I'm sure I'm missing something simple due to lack of experience.
Thanks!
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On 7/29/2016 2:25 PM, DerbyDad03 wrote:

Consider also not using a guide bushing at all. I use a jig the exact size that I want to cut and then use a top bearing flush cut bit. The top bearing rides along the jig sitting on top of the work. And you only need one size bit. Use a jig with a wider opening for the wider mortise. Obviously the wider mortise will take a pass or two down each side of the jig opening.

There are a bunch of different bushings for different kinds of jigs. Think DT jigs, Box joint jigs etc. And then there are special bushing for inlay, cutting the indention and cutting the inlay to fit snugly inside that indention.
Also do not assume that bits will be centered in the bushing. Typically the bushing is not centered with the bit. This shows up if you clock the router while making a pass. This can be adjusted by centering the bushing with the collet with a special cone shaped bit but only if you have the ability to center the bushing.
And this is exactly why I do not us a bushing vs. a 1/2" diameter top bearing flush cut bit. for cutting mortises.

Live and learn. Some of us have been down this road, some have not yet.

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On Friday, July 29, 2016 at 6:33:48 PM UTC-4, Leon wrote:

Thanks for the tips, but...
Are you suggesting 2 jigs vs. 2 bits and a guide bushing for my situation - one jig for the 5/8" mortise, requiring multiple passes with the 1/4" bit and then one jig for the 1/4" mortise that needs to be centered in the 5/8" mortise?

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Yes. But good luck finding a top bearing flush cut bit in 1/4" size. Does the centered mortise have to be 1/4" wide? Could it be 1/2"? And or since the centered smaller mortise will not be seem could you free hand it with a plain straight cut 1/4" bit after making the bigger mortise?

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On Saturday, July 30, 2016 at 8:59:31 AM UTC-4, Leon wrote:

Yes to both suggestions regarding the smaller mortise.
I'll play with the 5/8 bushing this weekend, since I already have it and see what happens. If it's a problem, I'll look at the top bearing option.
BTW - the bushing adapter I bought on eBay fits perfectly. There's just a tiny bit of wiggle before the screws are tightened which seems to allow for some centering capability. Of course, I don't have a centering cone...yet.
http://www.ebay.com/itm/Router-Template-Guide-Bushing-Apapter-Plate-for-Makita-Freud-Hitachi-/111153232468
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On 7/30/16 8:00 AM, DerbyDad03 wrote:

If you have a plunge base, often you can use a 45 degree bevel bit for centering (the plunge allows you to remove the bit after centering your bushing). For a fixed base, often you can use a dovetail bit for centering.
-BR
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On 7/31/2016 9:32 AM, Brewster wrote:

Two flute bits can get you close but the bushing can still end up slightly off center.
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On 7/29/2016 2:25 PM, DerbyDad03 wrote:

Basically what Leon said.
As long as you make a jig that will have stops to limit travel (generally in two dimensions), then you really don't need a guide bushing.
However, guide bushing can be critical when cutting irregular shapes and patterns, like dovetail joint pins and tails. AAMOF, the correct size of the bushing when using commercial jigs, like dovetail jigs, is extremely critical to the specific jig.
And yes, centering is difficult to achieve, but can be mitigated by always holding the router in the same orientation when routing, regardless of the direction you move the router.
For a straight cut the same size as the bit, just design a jig with stops at the proper project length, as you see here:
http://e-woodshop.net/Jigs.htm
<scroll down to "Router Mortising Jigs">
Depending upon your router, you may have to capture the router in the jig so it is fixed on the long axis of the cut, then use spacers as stops to vary the length of the cut.
The photos should make it obvious ...
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On Friday, July 29, 2016 at 7:42:32 PM UTC-4, Swingman wrote:

...and worth at least a thousand words. Thanks!
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For one reason, different size bushings let you offset different distances form whatever the bushing is guiding on; could be handy if you want parallel grooves and have the tight size bushings.
There are only two times I've used bushings:
1. When using a router to make shelf pin holes.
2. When I need to follow a curved surface.
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On Saturday, July 30, 2016 at 11:01:02 AM UTC-7, dadiOH wrote:

For making sliding-dovetail slots, one requires a guide jig. So, cobble something up quickly, and make the guide slot with your dado set on the table saw... but, that slot won't be wide enough to guide a router base, so unless you have dovetail bits with guide bearings (I don't), just use the bushings. Centering doesn't matter, here, if you don't rotate the router during the cut.
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On 7/30/2016 4:09 PM, whit3rd wrote:

Actually I made sliding DT joints with my router table and fence. No jig.
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On Saturday, July 30, 2016 at 9:11:39 PM UTC-7, Leon wrote:

I'm making dovetail slots across a 7' board, for tall bookshelves. Can't get it accurate enough on the router table, clamping a guide works. Biggest problem, is getting it clamped without interfering with router movement.
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I've had a few times where I've needed to stop partway through and move a clamp. Three clamps is the minimum for this: two clamps keep the piece from rotating as the third is moved.
Sometimes it's necessary to tack nail a board in place using fine brads. Norm used to do it all the time. (I suspect when Norm makes pancakes, he puts a "couple of brads in until the glue dries.") It's up to you to make them invisible once removed. (I've found small holes like that are often best not fixed.)
Puckdropper
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On 7/31/2016 12:47 AM, Puckdropper wrote:

LOL Call those holes "worm holes" for added value. Rockler has "Wormy" Red Oak on sale for $3.99 per BF. I can get perfectly fine Red Oak for less.
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On 7/30/2016 11:47 PM, whit3rd wrote:

Well there you only need a straight edge guide, I don't see the need for a guide bushing. The guide can be a hunk of plywood.
clamping a guide works. Biggest

As stated above, use a large enough piece of plywood as a strait edge guide.
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On 7/31/2016 10:29 AM, Leon wrote:

It also helps to cut a dado to remove most of the wood before running the DT bit.

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On Sunday, July 31, 2016 at 8:29:41 AM UTC-7, Leon wrote:

The slot isn't the size of one pass with the dovetail bit, though. It's nearly the full thickness of the thinnest of the shelf boards There's only about .003" tolerance, for the shelves to be a press-fit in the uprights. I'm actually using half-dovetails, one router with straight bit against the left edge, one with a dovetail bit against the right edge; that simplifies the shelf-end cuts.
The guide is plywood, but the guiding edge has a hard face applied; I tried steel, and quartersawn oak. Both work, but straight steel L-section is expensive (and if it's not-so-straight, you're back to seeking hardwood scraps for the next edge).
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